An adaptive stereoscopic-PIV system has been constructed for flow analyses around a cylinder body (length=60mm, diameter=10mm, polystyrene). The body is floated by the swirling flows in a vertical pipe with a length of 1100mm and an inner diameter of 100mm. Four optical sensors (LEDs) were installed onto the pipe wall to detect the posture of the cylinder body. The constructed stereoscopic-PIV system is activated by the signals of the optical sensors when the cylinder is in the upright posture. The measurement system consists of two cameras, a Nd-Yag laser, and a host computer. Flow characteristics around the floating cylinder body and the motional characteristics of the body itself are investigated when the body is in the upright posture. It has been verified that the motion of the floating cylinder becomes stable when the azimuthal velocity component (W component) of the swirl flow is maintained at stable states.